Cholesteric colour filter

ABSTRACT

A reflective filter structure (30) adapted to reproduce color by reflection, which filter structure comprises a first domain (31) reflecting visible red color at zero angle of incidence, as well as a second domain (34) reflecting invisible infrared color at zero angle of incidence. Furthermore, the second domain is adapted to reflect visible red color at a predetermined angle of incidence greater than zero. The invention also relates to a display device based on the reflective filter structure.

[0001] The present invention relates to reflective colour filters, andmore particularly to cholesteric filters for use in connection withdisplays devices. The invention also relates to a display deviceutilising such filters.

[0002] Cholesteric materials, also known as chiral nematic materials,can be used as reflective colour filters. One advantage of such filtersis that they do not absorb any light, in contrast to conventionalabsorbing colour filters. Thus, a higher filtering efficiency may beachieved when using cholesteric filters rather than conventional,absorbing colour filters. Furthermore, manufacturing of cholestericcolour filters is less complicated than manufacturing of absorbingcolour filters, in the respect that fewer processing steps are typicallyneeded. However, the colour reflected from cholesteric colour filters isstrongly dependent on the viewing angle (i.e. the angle of incidence onthe filter). The angular acceptance bandwidth of display devices of thereflective type based on cholesteric liquid crystals is typically around±20°, at most.

[0003] A layer of cholesteric material ordered in a planar state(principal axis perpendicular to the surface of the filter layer) actsas an interference film that reflects light of a wavelength matching thepitch of the cholesteric structure. Light not meeting the interferencecriterion is transmitted through the material. Thus, in order to reflectone specific wavelength, the cholesteric material must have a pitchmatching that specific wavelength.

[0004] When the angle of incidence is greater than zero, i.e. at anoblique angle of incidence, the effective pitch of the cholestericmaterial is changed. By consequence, the colour reflected is alteredaccordingly by the angle of incidence, as mentioned above. Thewavelength λ_(max) for which maximum reflection occurs is given by:

λ_(max) =n _(avg) p cos(α),

[0005] where n_(avg) is the average refractive index of the material, pis the pitch of the cholesteric structure and α is the angle ofincidence with respect to the layer normal. It should be noted that, inthe above equation, refraction at the layer surface between thecholesteric material and the ambient is neglected.

[0006] Clearly, as the angle of incidence increases, the wavelength ofthe light reflected decreases. In other words, when a cholesteric filteris viewed under increasing angles, the wavelength of reflected lightdecreases. For display devices, this angle dependency is verydisturbing. For the red component of a multi-colour display, this isparticularly annoying, since the colour turns yellow or green. In thecontext, it should be noted that the human eye is most sensitive forgreen light. Therefore, the change from red towards green becomes moreapparent when a colour display based on cholesteric filters is viewedunder an oblique angle. Also, the reproduction of red colour isdeteriorated, leading to a loss of colour space.

[0007] One previously proposed solution to overcome the above-describedproblem is to introduce an absorbing filter, which absorbs the unwantedcolours. However, such an approach to the problem has severallimitations. Firstly, it reintroduces the absorbing colour filters. Asmentioned above, one advantage of using reflective filters (such ascholesteric filters) is the possibility of avoiding the expensive andcomplicated absorbing filters. Secondly, an absorbing filter willinfluence other colours as well, thereby skewing the colourreproduction. A cholesteric filter layer comprising absorbing dyes whichabsorb unwanted colours is disclosed in WO 00/33129.

[0008] Hence, in the art of reflective, cholesteric colour filters,there is a need for improvements regarding the colour reproduction atoblique viewing angles.

[0009] It is a general object of the present invention to provide asolution to the above-described problems regarding the colour shift ofcholesteric filters when viewed under oblique angles. This object isachieved by a device and a method of the kind presented in the appendedclaims.

[0010] Hence, the present invention provides a reflective filterstructure arranged to reproduce colour by reflection, said structurecomprising a first domain adapted to reflect, at normal angle ofincidence, light having a red colour, said filter structure beingcharacterised by further comprising a second domain adapted to reflect,at normal angle of incidence, light having an infrared colour, whereinsaid second domain is further adapted to reflect, at a predeterminedangle of incidence greater than zero, light having a red colour.

[0011] In particular, it is an object of the present invention toincrease the acceptable viewing angle of colour display devices based onreflective filters comprising a material having a cholesteric order.

[0012] Thus, it is an object of the present invention to provide areflective colour filter based on cholesterically ordered liquidcrystals, which filter exhibits an enhanced colour reproduction atoblique viewing angles without the need to incorporate absorbingelements. Nevertheless, it can be advantageous to combine the featuresof the present invention with absorbing elements or dyes in order toachieve particular, desired effects.

[0013] The present invention is based on the recognition that theaddition of an infrared domain to a conventional RGB pattern of a matrixdisplay can be used for the reproduction of red colour when the displayis viewed under oblique angles. The infrared domain is adapted toreflect, when viewed under a normal angle of incidence, light within theinfrared range of the colour spectrum. Thus, under a normal (i.e.perpendicular) viewing angle, the infrared domain is invisible to thehuman eye. However, when viewed under an oblique angle, the wavelengthof light reflected from the infrared domain is shifted towards shorterwavelengths. Consequently, the colour reproduction from the infrareddomain can be arranged to fall within the red range of the colourspectrum when said domain is viewed under an oblique angle. By virtue ofthe infrared domain becoming visible red when viewed under obliqueangles, the defective yellow or green colour is compensated.

[0014] Although the invention is described by means of examplesemploying cholesteric liquid crystals, it is to be understood that othertypes of reflective colour filters are possible within the scope of theinvention.

[0015] One advantage of the reflective filter structure according to thepresent invention is that the additional infrared domain can bemanufactured with existing technology. The colour reflected by acholesteric film is determined by the pitch of said film. Thus, aninfrared domain could easily be incorporated in a matrix display byproviding a domain of cholesteric material having a pitch that isdifferent from the pitch of the red, green and blue domains, forexample. The present invention gives important enhancement to reflectivecolour filters without introducing new and complicated processing steps.

[0016] Advantageously, the infrared domain is formed in a layer that iscommon to all domains, i.e. the domains are formed side by side in asingle layer. Alternatively, the infrared domain is formed in a secondlayer on top of the layer comprising the red, green and blue domains. Inthe latter case, the infrared domain is formed solely on top of the reddomain, leaving the green and blue domains uncovered. Similarly, theinfrared domain can also be formed in a second layer underneath the reddomain.

[0017] It should be noted that the terms “infrared”, “red”, “green” and“blue” domains refer to the colours reflected at a normal (i.e.perpendicular) viewing angle. However, the fact being appreciated thatthe reflected colour shifts to shorter wavelengths as the viewing angleincreases.

[0018] In one aspect, the present invention provides a reflecting filterstructure of cholesteric material, wherein the reproduction of redcolour is augmented for oblique viewing angles. To this end, areflective filter structure comprises both a red domain and an infrareddomain. At zero angle of incidence (i.e. under normal viewing angle),the red domain reflects light of red colour, and the infrared domainreflects light of infrared colour. At an oblique angle of incidence, theinfrared domain reflects light of red colour, while the red domainreflects light of yellowish-green colour. As an example only, theinfrared domain can be adapted to reflect light within the red range ofthe colour spectrum when the filter is viewed at an angle of 45°. Thus,the reproduction of red colour is enhanced at an oblique viewing angleby the presence of the infrared domain.

[0019] In another aspect, the present invention provides a pixelstructure for full colour reproduction, the pixel structure comprising ablue domain, a green domain, a red domain and an infrared domain.Preferably, the area of each domain is carefully selected in order toreproduce the best possible colour space. Different selections of domainsizes and domain layout will be further described in the followingdetailed description of some preferred embodiments of the presentinvention.

[0020] Manufacturing of patterned layers of cholesteric material isknown in the art. For example, WO 00/34808 discloses a method ofmanufacturing a patterned layer of a polymer material having acholesteric order, in which the material is oriented is such a way thatthe axis of the molecular helix of the cholesterically ordered materialextends transversely to the layer.

[0021] Further features and objects of the present invention will beappreciated when the following detailed description of some preferredembodiments thereof is read and understood. In the detailed description,reference is made to the accompanying drawings, on which:

[0022]FIG. 1 schematically shows a prior art filter layer ofcholesterically ordered material;

[0023]FIG. 2 schematically shows a reflective filter structure accordingto the prior art;

[0024]FIG. 3 schematically shows a first embodiment of the presentinvention;

[0025]FIG. 4 schematically shows a second embodiment of the presentinvention;

[0026]FIG. 5 schematically shows a third embodiment of the presentinvention;

[0027]FIG. 6 schematically shows a fourth embodiment of the presentinvention;

[0028]FIG. 7 schematically shows a fifth embodiment of the presentinvention; and

[0029]FIG. 8 shows a plot of colour reproduction by reflective filtersaccording to both prior art and to the present invention.

[0030] By way of introduction, the improvements of reflective colourfilters achieved by the present invention will be described by way of anillustrative example. A layer of chiral nematic liquid crystals 10 (i.e.a cholesteric material) is schematically shown in FIG. 1.

[0031] Consider a reflective colour filter having a first domain ofcholesteric material adapted to reflect, at normal angle of incidence,light having a red colour. The wavelength of light perceived as redcolour is in the range from 600 nm to 700 nm, typically around 650 nm.The pitchp of the cholesteric material required for reflecting light ofthe centre wavelength λ at normal incidence (α=0) in a material ofaverage refractive index n_(avg) is given by p=λ/n_(avg). For non-zeroangles of incidence, the wavelength of reflected light is shiftedtowards shorter wavelengths (i.e. the wavelength of peak reflectivity isshifted towards shorter wavelengths). If refraction at the interfacebetween the cholesteric layer and the ambient (here assumed to be airhaving a refractive index of one) is accounted for, the centrewavelength reflected at an angle of incidence α is given by

λ(α)=λ₀ cos[sin⁻¹(n _(avg) ⁻¹sin(α))],

[0032] where λ₀ is the wavelength reflected at zero angle of incidenceand n_(avg) is the average refractive index of the cholesteric material.

[0033] If said first domain of cholesteric material is adapted toreflect light having a wavelength of 650 nm at normal angle ofincidence, the wavelength for which maximum reflection occurs at anoblique angle of a is given by the equation above. For example, at aviewing angle of 45° and an average refractive index n_(avg)=1.5, thereflected light will be centred around 573 nm (which is yellow).

[0034] Now, by juxtaposing, in the same layer, said first domain with asecond domain of cholesteric material that is adapted to reflect, atnormal angle of incidence, light in the infrared region of the spectrum,the reproduction of red colour at oblique angles is greatly enhanced. Ifthe second domain is adapted to reflect light having a wavelength ofabout 740 nm at normal angle of incidence, the reflected light at 45°angle of incidence will be centred around 650 nm. Consequently, the lossof reflection at 650 nm from the first domain is compensated byreflection at 650 nm from the second domain. Hence, the perceived colouris a mixture between red and yellow, rather than pure yellow.Preferably, the sizes of the first and the second domain, as well as theseparation there between, are small enough for the two domains not to beresolved by the unaided human eye.

[0035] Alternatively, the infrared domain can be formed on top of thered domain, in an additional layer. In this case, the infrared domain isprovided solely on top of the red domain, leaving the green and bluedomains uncovered. Similarly, the infrared domain can also be arrangedunderneath the red domain in a straightforward way.

[0036] It is envisioned that the present invention will have its primaryfield of use in colour display devices, such as RGB matrix displays. Aprior art structure of an RGB display is illustrated in FIG. 2. Anypixel 20 in the display comprises three sub-pixels: a red sub-pixel 21,a green sub-pixel 22 and a blue sub-pixel 23. Each sub-pixel 21, 22, 23is comprised of a domain of cholesteric material adapted to reflect, atnormal angle of incidence, a predetermined wavelength of light, i.e.red, green and blue light, respectively. In order to provide anupdateable pixel, the reflectivity of the cholesteric material can beswitched on and off, or adjusted, electrically.

[0037] However, as mentioned above, the angle of acceptance for a decentcolour reproduction is only about ±20°.

[0038] According to the present invention, increased viewing angles areachieved by introducing into the pixel also an additional sub-pixel thatis adapted to reflect, at normal angle of incidence, light having aninfrared colour. Consequently, at oblique viewing angles, the reflectionfrom this additional sub-pixel will shift towards visible red.

[0039] In FIG. 3, a plan view of a first preferred embodiment of thepresent invention is shown. In this case, the conventional red domain isdivided into one red domain 31 and one infrared domain 34. The shownembodiment is probably the most straightforward to manufacture, with aminimum of alterations of the process. However, the reproduction of (thebrightness or reflection coefficient for) red colour is lower than thereproduction of green and blue, since the surface areas of the red 31and the infrared 34 domain, respectively, are smaller than the surfaceareas of the green 32 and blue 33 domains. In some cases, this might beacceptable or even desirable. In other cases, however, it may benecessary to take measures in order to avoid the difference in colourbrightness, such as the examples presented below.

[0040] In FIG. 4, a plan view of a second preferred embodiment of thepresent invention is shown. Here, the intensity of red, blue and greenlight is balanced by each of the infrared 44, the red 41, the green 42and the blue 43 domains having substantially equal surface areas. As theviewing angle increases, and as the reflection of red light from the reddomain 41 decreases, the reflection of visible red from the infrareddomain 44 increases accordingly. It is to be understood that the area ofeach domain 41, 42, 43, 44 can be designed so that the desired colourbrightness is achieved.

[0041]FIG. 5 schematically shows a plan view a third preferredembodiment of the present invention. In this embodiment, the red R andthe infrared IR domains are interspersed in a checkerboard structure.The checkerboard structure enhances the mixture of the light reflectedfrom the red domain and the light reflected from the infrared domain.

[0042] In FIG. 6, a plan view of a fourth embodiment of the presentinvention is shown. This embodiment is similar to that shown in FIG. 5.However, in order to further enhance the reproduction of red colour, thecheckerboard structure of the red R and infrared IR reflective domainsis larger than in the previous embodiment. By choosing the surface areasof each domain (infrared, red, green and blue) appropriately, thereproduction of red colour can be balanced to the reproduction of greenand blue colour.

[0043]FIG. 7 schematically shows a side view of an embodiment of thepresent invention, in which an infrared IR domain is arranged in asecond layer on top of the layer comprising the red R, green G and blueB domains. The infrared IR domain is provided only over the red Rdomain, effectively leaving the green and blue domains uncovered. Forpractical reasons, also the green and blue domains may be covered, butwith a non-active NA domain that is essentially non-reflecting. It is tobe understood that the effect of the infrared IR domain on the colourreproduction of the green G and blue B domains should be avoided to thegreatest possible extent.

[0044] Although the infrared domain is shown to be provided on top ofthe red domain in FIG. 7, it is also possible to have the infrareddomain underneath the red domain.

[0045] The improved colour reproduction at increasing viewing anglesachieved by the present invention is illustrated in FIG. 8. The figureshows, in a colour triangle plot, the colour reproduction of a singledomain red filter (squares), and a dual-domain, red and infrared filter(triangles). In the colour triangle, the right corner corresponds to redcolour, the upper corner to green colour and the lower/left corner toblue colour. The red domain had a 100 nm wide reflection band centred at650 nm, and the infrared domain had a 100 nm wide reflection bandcentred at 750 nm. In the dual-domain filter, the surface area of thered domain was equal to the surface area of the infrared domain. Thecolour of the reflected light is plotted for viewing angles in air(outside of the colour filter) ranging from zero angle of incidence to60° angle of incidence, in steps of 5°. The figure clearly illustratesthat the reproduction of red colour is greatly enhanced when adual-domain filter according to the present invention is utilised.

[0046] Hence, by including an infrared domain according to the presentinvention, i.e. a reflective filter layer domain reflecting light withinthe infrared range of the spectrum when viewed from zero angle ofincidence and reflecting visible red when viewed from an oblique angleof incidence, in a reflective colour filter based on chirally nematic(cholesteric) liquid crystals, the colour reproduction at oblique anglesis considerably enhanced.

1. A reflective filter structure arranged to reproduce colour byreflection, said structure comprising a first domain adapted to reflect,at normal angle of incidence, light having a red colour, said filterstructure being characterised by further comprising a second domainadapted to reflect, at normal angle of incidence, light having aninfrared colour; and to reflect, at a predetermined angle of incidencegreater than zero, light having a red colour.
 2. A reflective filterstructure as claimed in claim 1, further comprising a third domainadapted to reflect, at normal angle of incidence, light having a greencolour.
 3. A reflective filter structure as claimed in claim 1 or 2,further comprising a fourth domain adapted to reflect, at normal angleof incidence, light having a blue colour.
 4. A reflective filterstructure as claimed in any one of the preceding claims, wherein atleast one of the reflective domains comprises cholesteric liquidcrystals.
 5. A reflective filter structure as claimed in claim 1,wherein the first domain is interspersed with the second domain in acheckerboard pattern.
 6. A reflective filter structure as claimed inclaim 1, wherein the reflective surface area of the first domain issubstantially equal to the reflective surface area of the second domain.7. A reflective filter structure as claimed in claim 3, wherein thereflective surface area of each domain is essentially equal.
 8. Areflective filter structure as claimed in any one of the precedingclaims, wherein each domain is formed in a single layer.
 9. A reflectivefilter structure as claimed in any one of the claims 1 to 7, wherein thesecond domain is formed in a second layer provided on either side of alayer comprising the first domain.
 10. A liquid crystal colour displaydevice of the reflective type, in which each pixel comprises a domain ofcholesteric material adapted to reflect, at normal angle of incidence,light of red colour, a domain of cholesteric material adapted toreflect, at normal angle of incidence, light of green colour, a domainof cholesteric material adapted to reflect, at normal angle ofincidence, light of blue colour, the display device being characterisedby further comprising an additional domain of cholesteric materialadapted to reflect, at normal angle of incidence, light of infraredcolour; and to reflect, at a predetermined angle of incidence greaterthan zero, light having a red colour.